1. Field of the Invention
This invention relates to non-invasive approaches for determining cardiac output in patients, specifically to re-breathing techniques for determining cardiac output in patients, and most particularly to methods and devices for setting the volume of gases to be re-breathed by a patient.
2. Statement of the Art
It is frequently desirable to monitor a patient""s cardiac output during medical and surgical procedures. Non-invasive cardiac output (NICO) techniques are rapidly gaining favor. Re-breathing methods are particularly advantageous because they are non-invasive, use the accepted Fick principle of calculation, are easily automated, allow cardiac output to be calculated from commonly monitored clinical signals, and, if a partial re-breathing technique is used, require no patient cooperation.
In re-breathing methods, a modified form of the Fick Equation may be used to estimate cardiac output from observed changes in expired CO2(xe2x80x9cpCO2xe2x80x9d), which corresponds to arterial CO2. A comparison is made between a xe2x80x9cstandardxe2x80x9d ventilation event and a sudden change in ventilation which causes a change in expired CO2 values and a change in excreted volume of CO2. The sudden change in ventilation is accomplished by causing the ventilated patient to re-breath a specified amount of previously exhaled air.
Conventional re-breathing circuits provide only a fixed re-breathing volume, which may not be optimum, or even suitable, for patients of various sizes and respiratory capacities and resistances to minimize or eliminate interference with the ventilator function. It is also known to employ a variable volume for re-breathing, as disclosed and claimed in U.S. patent application Ser. No. 08/770,138, filed Dec. 19, 1996, assigned to the assignee of the present invention and the disclosure of which application is hereby incorporated herein by this reference. One preferred approach to providing a variable volume for re-breathing as disclosed in the aforementioned application is to employ a variable-length (and thus volume) loop of tubing connected at both ends to a ventilator or breathing circuit by means of a so-called airway valve. Particularly suitable airway valve configurations are disclosed and claimed in U.S. patent application Ser. No. 09/173,517, filed Oct. 15, 1998, assigned to the assignee of the present invention and the disclosure of which is incorporated herein by this reference. It would be advantageous to have a convenient method for setting the volume of such a tubing loop to various predetermined values corresponding to different patient respiratory parameters. Pre-setting the tubing loop volume is advantageous because it minimizes the adjustments required after the tubing loop is in use. In some cases it would also be desirable to provide protection and support for the variable-length tubing loop and associated structures, such as the airway valve and various actuation and monitoring tubing employed during re-breathing, during shipping and handling. It is further desirable to provide instructions for correct use and adjustment of the variable length tubing loop displayed prominently and connected directly to the tubing loop, to minimize the potential for incorrect use of the device. It may also be advantageous to provide instructions for use of monitoring or other equipment used with the inventive device. Finally, it would be desirable for the device used to set the volume of the tubing loop to add little or no additional equipment to the re-breathing system. These and other advantages are provided by the present invention.
The present invention provides a method and device for conveniently and economically setting the volume of a variable-length tubing loop used in a re-breathing circuit to various predetermined values. The device includes a card of non-elastic, disposable material which is attached to the a portion of the tubing loop or to an adjacent structure and which has marked on it indicia corresponding to several positions to which the tubing loop can be extended in length to set the volume of the loop at different, selected, predetermined values. The card may be configured to fold around the tubing loop and adjacent structures to provide support and protection during shipping and handling of the assembly. The card preferably has instructions for correct use and adjustment of the tubing loop printed on it. Instructions for use of a monitoring device, or other instructions may be printed on the card as well. Some or all of the card may be removable from the tubing loop so that once the desired tubing volume is set, a portion or all of the card may be detached and disposed of.